The present invention relates to a pressurizer, and in particular a pressurizer for the primary coolant system of a nuclear reactor of the pressurized water type. More specifically, the present invention relates to a passive system for providing both a pressure release and a quenching spray of liquid coolant in the pressurizer to suppress the pressure when it reaches a predetermined value.
In a nuclear reactor of the pressurized water type, the primary coolant system generally includes a plurality of primary coolant circulation loops, each containing a steam generator and a coolant pump, which are connected to the reactor vessel wherein they are interconnected. To suppress boiling of the primary coolant, which is generally water, a pressurizer is connected to the primary coolant system so as to maintain a desired pressure or pressure range within the primary coolant system. This pressurizer, which is a closed vertical vessel having a coolant connection between its lower end and the hot leg of one circulation loop of the reactor coolant system, is provided with heaters in its lower end for heating the coolant within the pressurizer sufficiently to convert the coolant to a vapor or gas which is trapped in the upper portion of the pressurizer vessel so as to maintain a pressure of for example 150 atm in the primary coolant system. The vapor space in the top of the pressurizer also provides an additional volume for expansion of the liquid coolant upon an increase in its temperature, for example as a result of a decrease in load.
To limit the maximum pressure in the pressurizer, and thus in the reactor coolant system, and/or to reduce the pressure when necessary, a number of devices are conventionally used in the upper end of the pressurizer. Initially, the upper end of the pressurizer vessel is provided with at least one spring loaded safety valve, which is responsive to the pressure in the upper end of the pressurizer reaching a predetermined maximum safe value, which vents the gaseous coolant, e.g. steam, in the upper end of the pressure vessel to a pressurizer relief tank wherein it is condensed and cooled by mixing with liquid coolant that is near ambient temperature. Moreover, to permit relief of the pressure in the upper end of the pressurizer vessel before it reaches the maximum pressure to which the safety valves are set, and thus limit any pressure excursions occurring during normal operation, one or more power operated relief valves are connected between the upper end of the pressurizer vessel and the pressurizer relief tank. These power operated pressure relief valves are controlled by means of a pressure sensor which senses the pressure in the upper end of the pressurizer, and opens the pressure relief valves at a pressure slightly lower than the pressure to which the safety valves are set. For example, with the safety valves set at 2485 psig, the power operated relief valves could be set to 2335 psig.
Finally, in order to control and/or suppress the pressure in the pressurizer vessel, the upper end of the pressurizer is provided with a spray nozzle which is connected via controllable valves to the cold leg of one or more of the circulation loops of the primary coolant system so as to spray liquid coolant onto the steam and condense same. The rate of spray is selected such that following a step load reduction in power of a given small percent of full load, the pressure in the pressurizer will not reach the operating or set point of the power operated relief valves.
Although the above described system operates satisfactorily, problems do occur, particularly with regard to the power driven relief valves. Such valves intially have a tendency to leak, thus resulting in the need for further safety measures in these lines as well as increased maintenance costs. Additionally, the opening of such valves results in a quantity of the primary coolant leaving the otherwise closed primary coolant system, and this released quantity of coolant must later be resupplied to the primary coolant system. However, since these relief valves vent the pressure to a pressurizer relief tank, which also receives liquids from other relief valves in the reactor system, the cooling fluid in the pressurizer relief tank must be reprocessed before it can be returned to the primary coolant system, and this processing step is clearly undesirable.